In computed tomography (CT) and fluoroscopy imaging systems, X-ray radiation spans a subject of interest, such as a human patient, and a portion of the radiation impacts a detector or a photographic plate where the image data is collected. In some X-ray systems the photographic plate is then developed to produce an image, which may be used by a radiologist or attending physician for diagnostic purposes. In digital X-ray systems a photo detector produces signals representative of the amount or intensity of radiation impacting discrete pixel regions of a detector surface. The signals may then be processed to generate an image that may be displayed for review. In CT systems a detector array, including a series of detector elements, produces similar signals through various positions as a gantry is displaced around a patient.
In the images produced by such systems, contrast is created based upon the varying attenuation of the X-rays by the materials encountered as the X-rays penetrate the patient's tissue. Typically, materials having atoms with a greater atomic number will have a greater attenuation of X-rays. Thus, tissues such as bone may create relatively high contrast within an image compared to other tissues, such as fatty tissue. Some techniques used for CT and fluoroscopy imaging use a contrast agent to artificially create contrast within an area that would typically not have relatively high contrast, such as blood vessels. The contrast agents may include one or more atoms capable of attenuating X-rays with a relatively high degree of efficiency, such as iodine. For example, in CT angiography, a contrast agent is typically injected into the patient, followed by CT imaging. The contrast agent typically perfuses through certain tissues of the patient, and the resulting CT images contain regions of enhanced contrast corresponding to the areas that are perfused with the contrast agent.
Contrast agents may include ionic or non-ionic, organic compounds. Contrast agents that utilize iodine typically include water-soluble organic compounds based on their relatively low toxicity and their covalent bonding of iodine atoms. Thus, in addition to the iodine atoms, such contrast agents include carbon (C), hydrogen (H), and may include nitrogen (N), oxygen (O) and other atoms having relatively low X-ray attenuating properties. Accordingly, the proportion of these atoms to iodine atoms may affect the signal response of the contrast agent to the X-rays. Thus, the use of different contrast agents may lead to varying signal responses based at least on the molecular structure of the contrast agent.